1. Field of the Invention
The present invention relates generally to heavy density non-metallic honeycomb structures. More particularly, the present invention involves increasing the formability of such honeycomb structures so they can be made into a wide variety of non-planar shapes.
2. Description of Related Art
Honeycomb structures which include bisector sheets are generally referred to as "high density honeycomb". These types of reinforced honeycombs are usually composed of a stack of alternating corrugated and bisector sheets which are glued or otherwise bonded together. A portion of a typical high density honeycomb is shown at 10 in FIG. 5. The honeycomb 10 includes bisector sheets 12 and corrugated sheets 14 which are bonded together at node junctures 16. As can be seen from FIG. 5, the bisector sheets 12 split the hexagonal honeycomb cells down the center. This configuration adds density, strength and bonding surface to the core. The high density honeycombs are well-suited for use in situations where high structural strength is required. However, the inherent stiffness of high density honeycomb and the presence of the bisector sheets makes it difficult to form such structures into non-planar shapes without damaging the honeycomb.
As shown in FIG. 5, honeycombs are three dimensional structures which are characterized as having a thickness (T direction) which is measured parallel to the honeycomb cell and provides a measure of the honeycomb depth. The width (W direction) of the honeycomb is measured perpendicular to the T direction and provides a measure of the height of the stacked honeycomb cells. The length (L direction) of the honeycomb is measured perpendicular to both the T and W directions and provides a measure of the length of the corrugated and bisector sheets present in the honeycomb (see FIG. 5).
When forming non-planar high density honeycomb structures, planar honeycombs of the type shown in FIG. 5 are formed in the L and/or W directions by applying heat and/or pressure to the honeycomb. When forming in the L direction, the outside radius of the core must expand in the L direction and inside radius of the core must contract in the L direction. The bisector sheet passing through the cell will not allow the cell to expand on the top of the radius. As a result, the inside of the cell must condense more. This causes the inside cell to deform or crush to such a degree that the resulting core may have reduced strength and/or the corrugated and bisector sheets may separate at the node junctures.
When forming non-planar high density honeycombs in the W direction, the outside radius of the cell must expand in the W direction and the inside radius of the core must contract in the W direction. The bisector sheets limit the movement of the cell walls so that the usual result is that the relatively stiff node junctures are torn apart.
Various approaches have been taken to try and increase the formability of high density honeycombs. For example, attempts have been made to increase node strength by using higher strength adhesives. Various thermosetting resins have been used in the resin matrix of composite honeycomb walls to enhance heat formability and various thermosetting dip resins have been used to coat honeycomb walls. The use of hybrid weaves for composite honeycomb walls has also been proposed. Although all of these approaches have achieved some improvement in formability of high density honeycomb, there still is a continuing need to increase and enhance the formability of such honeycomb structures.